The development of cataracts and other opacities in the ocular media of eyes of persons is a well known. A cataract constitutes an opacity of the eye's lens that functions to scatter, disperse or absorb light and thereby obscures vision. To date, the accepted remedy for this affliction requires the surgical removal of the cataractous natural lens and compensating for its refractive power by various optical means.
This surgical technique, although normally corrective, can give rise to a degree of risk, particularly in elderly persons. The advancement of age also makes a person prone to other retinal-neural diseases and degenerative processes affecting vision, e.g., senile macular degeneration. Since a cataract obscures a clinician's view of a patient's retina and optic nerve head, as well as a patient's view of a conventional test stimulus, the detection and diagnosis of retinal disease is made correspondingly more difficult.
Prior to removing a cataract, a surgeon will seek assurance that doing so will improve the patient's vision. However, adequate assurance of this kind has proven difficult to obtain in many cases. It is thus a prevalent problem in the art and a primary object of this invention to provide a surgeon with an instrument for evaluating the functional integrity of a patient's retina and post-retinal visual pathways (retinal-neural level) before surgically removing opacities of the ocular media.
The following discussions relate to various present day instruments and test procedures for providing a surgeon with this type of pre-surgical information. Any adopted test procedure will normally recognize that several optical and physiological requirements must be satisfied to allow for adequate visual acuity. Such requirements include good retinal image quality, and intact retinal and post-retinal neural function.
A defect in the eye's optics, while resulting in poor resolution acuity, need not, however, preclude an evaluation of the neural pathways. Several commonly available test procedures are presently used clinically, but oftentimes fail when they are needed most. In particular, such test procedures tend to fail in those patients wherein the media opacity is very dense or a "window" is not present in a cataract to facilitate visual inspection of the retinal area.
One such test procedure constitutes the optical projection technique known as "potential acuity meter" (PAM). The procedure utilizes a device attached to a clinician's biomicroscope that projects a Snellen acuity chart (a standard eye chart) through a "window" or opening in an opacified eye lens. The patient simply reads the eye chart as though the patient were seeing it projected upon an external screen or wall. Unfortunately, although this technique is quite simple to use, it is only most successful in cases where the opacities are mild and will fail completely when no "window" is defined through the cataract.
Another test procedure constitutes the use of interferometry to test the visual potential of cataract patients. A laser beam or other suitable source of coherent light is split into two beams which are directed either through one or two "windows" in a cataract to produce an interference grating pattern within their overlapping retinal projection area. A patient's ability to resolve a sufficiently fine grating is interpreted as evidence of normal foveal function. Under certain conditions, however, interference acuity measurements may mislead the clinician. For example, if the grating pattern subtends a reasonably large retinal area (e.g., such as the five degree fields typical of some of these instruments), and if the stimulus is very bright (commonly the case), then visual performance of the parafoveal retina will be enhanced, and thus lead to an expectation of too good visual performance after surgery.
Thus, patients with macular dysfunction may exhibit relatively good interference grating acuity (e.g., equivalent to 20/30 to 20/60) even when their foveas are not functioning properly. In patients with macular degeneration, interference acuity may overestimate letter acuity, perhaps for this very reason. In contrast, when the required "window" or "windows" in the opacity are absent, the quality of the stimulus that is produced on the retina may be so compromised so as to preclude measurement of the patient's visual acuity, despite the presence of a healthy and normally functioning fovea.
A third test procedure constitutes electrophysiological tests wherein a patient's response to bright flashes of diffuse light [e.g., measurements of the electroretinogram (ERG) or visually-evoked cortical potential (VECP)] are recorded. This test is complicated by the fact that an ocular media opacity will function to scatter light from a "small-field" stimulus so as to cause it to fall upon a relatively large retinal area. In such cases, it may prove difficult to discriminate between responses that originate at the fovea (responsible for fine vision) and those originating in the surrounding retina. Patterned stimuli are, of course, compromised by the cataract itself and the detrimental effects of the cataract are difficult to uncouple from those which might be attributed to underlying retinal dysfunction.
The environment in which the test is performed, the type and condition of electrodes used, the amplification and filtering characteristics of the amplifier(s) (whether or not a notch filter or artifact rejection is used and, if so, the characteristics of the filter and limits of the accepted signal range), may all affect the data required for consideration and evaluation.
Tests which depend upon a subject's response to laser speckle patterns are also influenced to some degree by certain optical characteristics of the eye that are difficult to quantify in vivo. Even in those cases when the stimulus itself is exempted from consideration, valid interpretation of data obtained from these electrophysiological tests may prove difficult.
As described hereinafter, the instrument embodying this invention is particularly adapted to take advantage of a visual hyperacuity testing procedure that overcomes many of the drawbacks of the test procedures discussed above. As described in applicant's co-authored publication "Hyperacuity: A Promising Means of Evaluating Vision Through Cataract" (Progress in Retinal Research, Volume 4, Chapter 3, published by the Pergamon Press, Oxford, N.Y., on Mar. 28, 1985), the term "hyperacuity" refers to a patient's ability to successfully perform any one of a group of visual tasks, each of which requires the discrimination of very fine differences in the spatial locations of two or more visual stimuli, such as spots or points of light. While the limit of the human ability to resolve stimuli (i.e., to appreciate them as separate entities), corresponds rather well to the grain of the foveal photoreceptor matrix, the same stimuli typically need only be displaced from one another by about one-fourth this distance (or even less) for differences in their spatial positions to become apparent.
This extremely fine "hyperacuity" can be evidenced in a patients thresholds in detecting differences either in the relative positions of two lines or points (vernier acuity), in the apparent location in depth of stimuli that provides slightly different amounts of binocular image disparity (stereoacuity), or in appreciating the tilt of a single line (orientation discrimination). The robustness of the hyperacuities to variations in stimulus luminants and contrast leads to the conclusion that if a patient can reliably detect two stimuli, the patient can also compare their spatial locations with a high degree of accuracy. This characteristic of the hyperacuity test is important when considered in light of the fact that cataracts absorb, reflect, and scatter light.
Despite the fact that a cataractous eye receives an image of the external world that is more blurred, dimmer, and of lower contrast than that obtained via transplant optical media, good hyperacuity can still be expected, provided that the retina is healthy. The above publication further discusses the fact that the ability to recognize a Snellen letter (the standard eye chart method for testing vision) is more likely dependent upon a sharply focused retinal image than the ability to detect vernier misalignments. In particular, it is shown that a misalignment of the vernier targets is easily appreciated by a patient long after a comparable Snellen letter has been rendered unrecognizable due to blur.
To the extent that detecting misalignments of two stimuli during a hyperacuity test procedure is a simpler task than recognizing and identifying a figure so spatially complex as a Snellen letter, it can be concluded that measurements of a patient's hyperacuity will reveal more about the patient's visual function than the Snellen letter test, uncomplicated by considerations of the patient's ability or propensity to read and name letters. This desiderata of simplification is particularly pertinent for a patient population in which cerebrobascular disease and other health problems, comprising the "higher" mental functions, are frequently encountered.
The hyperacuity test for which applicant's instrument is especially designed is thus not limited by the enumerated drawbacks of the other test procedures briefly discussed above. First, no opening or "window" in the media opacity of a patient's eye is required to successfully conduct hyperacuity testing. Second, hyperacuity is so highly dependent upon retinal stimulus location that foveally and extra-foveally based responses are difficult to confuse when being compared; stimuli presented to a healthy fovea will unfailingly result in better performance than those presented to any extra-foveal location. The hyperacuities are also remarkably more robust to variations in stimulus luminants than those presented to any extra-foveal location.
The hyperacuities are also remarkably more robust to variations in stimulus luminants in contrast than are Snellen acuity and other indicies of visual resolution. These characteristics, when considered together with the resistance of the hyperacuities to stimulus blur, led applicants to recognize the potential clinical value of hyperacuity testing for evaluating retinal integrity behind ocular media opacities.
As further discussed in applicant's above-referenced publication, instrumentation was contemplated that only required two intense points of light whose relative positions could be controlled and ascertained with a high degree of precision. It was further suggested that the two points of light could be originated by use of an optically doubled laser beam for producing two-dot vernier stimulus. Applicant has incorporated these basic hyperacuity testing concepts into his instrument.